Ajax coder needed

I need an expert ajax coder to modify my code to work as stated and then explain to me how it all works. I have an online store that actually refresh the page everytime a product or set of products are choosen from a list OR a new category is choosen. When I select a number of products, the store catalog page sends 3 arrays to php code: products, price and quantity, each field in a different array. It updates the shopping cart and show the sumary box updated when the page refresh. When a category is selected, it refresh the page and show the products of category. All the information that is send to php are stored in a session variable, so I want to be sure that when we click on the checkout button of the store, it will have the right session variables already updated by ajax script or by the php ajax call. I want cart to update using ajax (i.e. without page refresh). I want category list box to update using Ajax. An expert coder should take no more than 4 hours to do this.